This invention concerns an electron beam cathodoluminescent panel display suitable for the display of television pictures. It is also useful for other image displays such as alphanumeric, computer and computer graphics.
The achievement of a feasible and practical flat panel television display has long been a goal of technologists in many parts of the world. However, to have widespread commercial significance, any such display must be technically and economically competitive with conventional cathode-ray picture tubes.
Such picture tubes are in an advanced state of refinement. In many respects, the attainable picture performance of the picture tube is at such a high level that there is little practical incentive for further technological improvemments. Contrast ratios, brightness levels, raster linearity, interlace and color field registration are quite acceptable to television viewers. Resolution, particularly in picture highlight areas, however, generally falls discernibly below theoretical system limits. This impediment is being overcome, however, by new types of high-resolution electron guns coming into use.
Conventional picture tubes do have characteristics that provide incentive to create a commercially viable alternative such as the flat panel display. For example, the largest color tubes commonly in current production have a display screen with an approximately 25-inch diagonal measurement, providing about 315 square inches of viewing area. The 25-inch measurement does not represent an absolute physical limit, but there are a number of practical considerations which rule out any major increase. Volume, weight and cost of the picture tube envelope tend to increase very rapidly for even modest increases in picture area. In addition, equivalent brightness and resolution are difficult if not impossible to attain in larger configurations.
In view of these disadvantages, the flat panel display represents a highly attractive alternative. An ideal panel display would provide picture performance equal to or exceeding the present quality levels of the picture tube, and would not be so rigorously size-limited.
A major effort in creating a flat panel display has been directed to the gas-discharge type; however, panels developed to date have not demonstrated adequate efficiency. In view of this fact, the efficiency of the electron beam of the picture tube in activating cathodoluminescent materials makes the use of such beams highly attractive in a panel display. Also, there is a wealth of readily available picture tube technology that is applicable to a panel display using electron beams; phosphor materials and application methods, high vacuum processing and fabrication techiques, and well-developed electron-optical design fondations are prime examples.
A significant drawback to the evolution of panel displays that can lend themselves to manufacture, and that can be economically mass-produced, is structural complexity. Panel displays are typically made up of an aggregation of discrete display elements each of which represents one unit of color information, or black and white information. Each display element may have components for electron beam guidance, channeling, directing, and accelerating, and, (in some cases) scanning the beam so that it selectively impacts one or more discrete cathodoluminescent phosphor targets, or, a photoluminescent compound. The display elements are typically separated by numerous spacers, usually insulative in nature. As a result of the multiplicity of its functions each display element in a cathodoluminescent display may in itself be quite complex in structure. This complexity is compounded by the fact that a quarter-of-a-million such display elements may be required in a black-and-white display, and as many as three-quarters of a million may be required for a color display. In view of the magnitude of such numbers, what may initially have seemed to be a relatively simple panel concept becomes staggering in structural complexity.
The problems of panel construction are compounded by the fact that the luminance output characteristics of all display elements must be substantially identical otherwise luminance non-uniformities readily perceptible to the eye can result. To achieve the necessary identicality in performance, the physical dimension of all display elements must in turn be substantially identical, and the passageways, whether for the conveyance of radiation or of particles, must also be substantially identical. Another troublesome requirement imposed upon spacer-support structures in many panel applications is that the display cell passageways be relatively deep, compared to their smallest lateral dimension. For example, the passageways in some applications necessarily must each have a front-to-back depth which is many times their narrowest width dimension.
Problems also beset the manufacture of said panel displays. Display panels are commonly fabricated by "stacking" elements such as beam-guiding electrodes, insulative spacers, and plate structures such as cathodes and anodes. In stacking such elements, tolerance build-ups may occur with the result that dimensions can vary intolerably across the length and width of the panel. Element forming and shaping methods may also present serious problems. Perhaps the most common method employed in fabricating such structures is by the use of photo-etching techniques. One of the problems attending the use of certain etching methods is that the etched material is "undercut" at a rapid rate. Inadequate dimensional accuracy and high cost also plague certain etching methods. None of the prior art panel structures have been found to be completely satisfactory. Most if not all have severe limitations in terms of their cost. Most are deficient in their ability to produce structures having passageways whose individual depth is greater than its smallest lateral dimension. Certain of these prior art approaches cannot meet the degree of accuracy, placement and configuration of the passageways which is required; other approaches fail when subjected to the severe thermal cycling operations which a panel must undergo during its fabrication. In short, there exists a very strong need for an improved structural component and electrode forming means in panel displays.
An attempt to utilize the electron beam in a flat panel display is shown by the "Aiken" tube (refer to FIG. 1) wherein a pair of electron guns 10 project beams 12 parallel to two enveloping plates 14 and 16, one of which is transparent. Beams 12 are diverted to fall upon opposite sides of cathodoluminescent surface 18. The beams are diverted by deflection plates 20 and 22, which are used to scan surface 18 in vertical and horizontal directions to produce an image. The concept is covered in a series of U.S. patents by Aiken, including U.S. Pat. No. 3,313,970. The beams are of high energy, and high potentials on the deflection plates are required to divert the beams toward the cathodoluminescent surface. Color rendition has also been less than ideal. Further, since the envelope is not self-supporting against atmospheric pressure, the concept would seem to be adaptable to only relatively small displays.
Gabor has disclosed a three-beam flat panel color display tube shown in highly simplified schematic form in FIG. 2. Three electron beams 24 are generated by electron gun 26, and turned back one hundred and eighty degrees around barrier 28 into an adjacent beam channel 30, where the beams are diverted again ninety degrees by electrodes 32 to impinge upon and scan cathodoluminescent color phosphor screen 34 through a shadow mask 36. This concept is covered in U.S. Pat. No. 3,171,056, among others. The Gabor tube is a very complex structure which must be made with extreme precision. Beam energies are relatively high, and high deflection potentials are required to scan the beam. It is believed that a complete operative tube has never been made. It is also thought that such a tube, if realizable, would be seriously effected by external influences such as the earth's magnetic field. Like the Aiken tube, it is not a self-supporting structure so its use would also be restricted to relatively small displays.
Charles, in U.S. Pat. No. 3,723,786, discloses a flat cathode-ray tube for direct viewing spot display of letters and numbers, as shown in simplified perspective form in FIG. 3. A longitudinal heater strip 38 comprising a series of thermionic emitters generates electrons which are formed into a series of electron beams 40 modulated by a succession of grids 42. The beams enter a space between two facing plates, one a backplate 44 having a series of horizontal strip electrodes 46 thereon, and the opposite plate a glass faceplate 48 having a conductive layer 50 and a cathodoluminescent material deposited thereon. The potentials on the strip electrodes 46 and the conductive layer 50 are made equal, resulting in "practically an equipotential space" (quoted from column 3, lines 23-24 of the subject patent). The beams travel through the space 52 to a collector electrode 54. Reducing the voltage on a conductive strip causes the potential to become unequal and results in diversion of the beams toward the faceplate at the level of the strip, according to the disclosure. The device as shown would seem to lend itself to only the simplest of displays. Again, such a display would necessarily be small as the structure is not self-supporting.
U.S. Pat. No. 4,028,582 to Anderson et al discloses a guided beam flat display device. The device comprises an evacuated envelope having a rectangular display section. A gun section is located on one edge of the display section. The display section includes a front and a back wall which are generally rectangular and in closely spaced, parallel relation. A plurality of spaced support walls between the front and back walls form the plurality of parallel channels. The gun section includes a gun structure for directing electrons into the channels. A beam guide in each channel confines the electrons in a beam and guides the beam along the length of the channel. The electron beam can be selectively deflected out of the guide at selected points along the guide to impinge upon a phosphor screen. A scanning deflector in each of the channel deflects the path of the beam as it passes from the guide to the phosphor screen so that each of the beams scan a portion of the screen.
For a color display device, three beams of electrons are preferably directed into each of the channels. In addition to modulating and scanning electrodes, line-sampling electrodees are included in each channel to generate an electrical signal which can be detected. The line-sampling electrodes can be used to detect the position and/or the intensity of the current of the beams.
By having each beam scan transversely across the portion of the phosphor screen in each channel, it is alleged that the number of beams necessary to achieve a scanning of the entire width of the device is reduced. It is stated, for example, that for display device forty inches in width having channel which are one inch in width, only forty beams for black and white, and forty sets of three beams for color, are necessary. The electrons comprising each of the beams are caused to remain in their respective channels by slalom focusing.
A major object of Anderson et al appears to be an attempt to reduce structural complexity through a reduction in the number of columns--forty in the example cited and a corresponding reduction in the number of beams. However, it is believed that the achievement in columnar structural simplicity is counterbalanced by the greatly increased complexity in beam functions, and the consequent complexity and difficulty in proper beam control.
Also relevant for their showing of flat display devices having guided beams are German patent disclosures 26 38 308 and 26 38 309. The '308 patent disclosure is the German counterpart of an application to Credelle, Ser. No. 607,490 referenced by Anderson et al. In the '309 patent, Osborne discloses an electron-beam-address device of flat design having means for the propagation of beams through channels. One means comprises an outer conductive tube, a conducting rod which extends down the axis of the tube, and a "spiraling" electrode which causes a beam of electrons to follow a spiraling path around and down the rod. Another means disclosed is the aformentioned "slalom-focusing" wherein charged wires or rods are arranged in a common plane between two parallel grounded, or negatively charged, plates. An electrostatic field causes an electron beam to follow a wavy path through the arrangement of rods or wires. Means are also disclosed for deflecting the beams out of the channels. Slalom focusing is described in a journal article titled "Slalom-Focusing," by J. S. Cook et al, Proc. of the IRE, November 1957, pp. 1517-1522. An electron gun for slalom-focusing systems is disclosed in U.S. Pat. No. 2,939,034 to Cook et al.
U.S. Pat. No. 4,067,994--Anderson, discloses a flat display with beam guides having substantially the same basic panel structure as the aforedescribed 4,028,582--Anderson et al. Means for beam confinement comprise a great many quadrupole electrode configurations extending along the path of the beam, which provide for a simultaneous focusing and defocusing of the beam in different planes to confine the beam. As with the aforedescribed Anderson et al disclosure, beam -diverting is caused by a change in potential on a selected one of a series of row-wise conductive strips, one for each row of the display, coated on the back wall. The resemblance to the beamdiverting means disclosed by Charles in the aforedescribed U.S. Pat. No. 3,723,786 will be noted.
In summing up, it appears that attempts to apply electron-beam picture tube technology to a flat panel display have been largely frustrated by one or both of such factors as the screen-size limitation dictated by the difficulty of providing internal envelope support in regions of beam excursion, and the need to utilize a high-energy beam to get adequate phosphor excitation. This need in turn dictates that beam control and modulating voltages be correspondingly high and out of the practical realm of utilization of transistor and integrated circuit technology. Structural complexity is another factor that has inhibited the realization of practical panel displays.